The present invention relates to an apparatus for separating a liquid in suspension, and more particularly to an apparatus for air/oil separation especially but not exclusively for use in gas turbine engine oil systems.
Oil systems, particularly gas turbine engine oil systems require separators for separating air and oil from the mixture produced during operation. These mixtures vary from oil emulsified with air to air contaminated by droplets of oil. For example, the compressed air streams used in gas turbine engines to pressure labyrinth seals for the engine main bearings in order to avoid excessive loss of a lubricating oil, almost invariably become contaminated with oil in the form of droplets suspended in the air. Contamination of the compressed air with oil mist is particularly disadvantageous because, loss of contaminated air from the labyrinth seals in the compressor causes fouling of the engine parts and produces noxious and unpleasant products in air drawn from the compressor for cabin pressurization. A further disadvantage is, of course, the increased loss of the lubricating oil from the engine oil reservoir, necessitating larger capacity reservoirs and adding to the weight of the engine. It has been a particular problem that even relatively small inefficiencies in such separators lead to the loss of a significant quantity of lubricating oil during each hour of operation of the engine.
Centrifugal separators have been extensively used in the aircraft industry in attempts to remove the majority of oil mist from compressed air streams. An example is described in U.S. Pat. No. 4,714,139 issued to Lorenze et al. on Dec. 22, 1987 in which an air/oil separator is used in a gas turbine power plant especially for an aircraft that includes a pump used for the separation of the air and the oil, out of the air/oil mixture. The pump is so constructed that centripetal and centrifugal forces are employed for the air/oil separation. The air passes by centripetal force through a sponge type filter structure and out through a hollow central shaft mounting the pump wheel or forming an integral one-piece component with the pump wheel. The oil does not travel through the filter but is reversed in its travel direction by centrifugal force for return into the lubricant circulating system.
Another example is U.S. Pat. No. 4,755,103 which issued to Streifinger on Jul. 5, 1988. Streifinger describes means for separating an air/oil mixture and for returning oil droplets to a circulation having oil consuming devices, which includes a porous air-permeable element built into the hollow main shaft between two oil impermeable walls whereby outer feed openings for the air/oil mixture are arranged in the upstream wall which correspond to openings in the main shaft while an air discharge opening is arranged through the downstream wall in its center which is in communication with the vent line. The oil is prevented from flowing out of the air discharge opening by centrifugal force and it is thrown back into the bearing chamber and is fed back to the tank by way of the suction line.
The disadvantage of the above prior art lies in that the oil separated from the air/oil mixture travels in a reverse direction relative to the air/oil mixture flow in the sponge type filter or the porous air-permeable element, and exits from the inlet for admitting the air/oil mixture, thereby creating a blockage and increasing the delta pressure across the air/oil separator, resulting in loss of efficiency.
Smith describes, in U.S. Pat. No. 4,049,401 issued on Sep. 20, 1977, a centrifugal separator for separating suspensions of oil mist in air and especially intended for use with aircraft engines. The separator comprises a rotatable chamber filled with a relatively rigid porous material and driven by a hollow shaft. An inlet for the suspension is provided in one end wall of the chamber and separate outlets for oil and air in the form of an apertured chamber outer wall and apertures in the shaft respectively. A buffer is positioned within the chamber between two different grades of porous material to facilitate radial movement of the oil droplets in the porous material. The oil droplets are directed away from the air/oil mixture flow path, thereby reducing the blockage and therefore the delta pressure across the air/oil separator. However, as pointed out by Smith, it has proven possible to dispense with the buffer and the differing grades of porous material without any significant loss of efficiency. In other words, the buffer and differing grades of porous material have not improved the efficiency of the separator.
Therefore, there is a need for an improved air/oil separator for more efficient separation of the air/oil mixture.
It is one object of the present invention to provide an air/oil separator with a relatively higher efficiency of separation.
It is another object of the present invention to provide an improved form of separator suitable for use in aircraft and lending itself to the construction of a modular and interchangeable separator that is simply constructed and relatively economical to manufacture.
In accordance with the present invention there is provided an apparatus adapted for separating a liquid suspended in a gas, comprising a rotatable vessel defining a chamber and a packing within the chamber adapted to rotate with the vessel. The rotatable vessel has an inlet at a first end of the chamber for admitting the gas having the liquid suspended into the chamber, an outlet for the liquid in the vessel in a radially outer part of the chamber and an outlet for the gas in the vessel in a radially inner part of the chamber at a second end thereof. The packing includes a substantially rigid matrix adapted to inhibit collapse under centrifugal forces during rotation of the vessel, the matrix including interstices defining a plurality of flow passages permeable to the liquid and gas for both axial and radial movement of the liquid and gas therethrough. A cut-away area is formed in the packing to facilitate ejection from the packing of liquid droplets formed in the passages of the packing, whereby the passages are relatively cleared to facilitate a continuous movement of a succeeding portion of the mixture of the liquid and gas therethrough. The cut-away area preferably extends inwardly and radially from an outer periphery of the packing. It is also preferable that the cut-away area is axially located close to the first end of the chamber, and axially aligns with the liquid outlet of the chamber.
In one embodiment of the present invention the cut-away area comprises a plurality of bores circumferentially spaced apart from one another, extending radially from the outer periphery of the packing.
In another embodiment of the present invention the cut-away area comprises an annular groove extending radially from the outer periphery of the packing.
In accordance with another aspect of the present invention, an air/oil separating arrangement is provided for a machine having a rotatable hollow shaft. In the arrangement a vessel defining a separating chamber is mounted to the hollow shaft and adapted to rotate together with the hollow shaft. The vessel has an inlet at a first end of the separating chamber for admitting a mixture of air and oil mist under a pressure differential between outside of the chamber and inside of the hollow shaft, an outlet in a radial inner part of the chamber in communication with the inside of the hollow shaft for exhausting the air into the hollow shaft under the pressure differential, and an outlet in the vessel in a radial outer part of the chamber for expelling oil droplets out of the chamber under centrifugal forces during rotation of the chamber. A packing is filled within the chamber adapted for rotation with the chamber. The packing has a substantially rigid matrix adapted to inhibit collapse under the centrifugal forces. The matrix includes interstices defining a plurality of flow passages permeable to the air and oil mist for both axial and radial movement of the air and oil therethrough. A cut-away area formed in the packing radially extending from an outer periphery of the packing to facilitate ejection from the packing of the oil droplets formed in the passages of the packing, whereby the passages are cleared to facilitate a continuous movement of a following portion of the mixture of the air and oil mist.
The cut-away area is preferably located axially close to but spaced apart from an end of the packing adjacent to the inlet. It is also preferable that the oil outlet is axially located close to the first end of the chamber and axially aligns with the cut-away area of the packing while the air outlet is axially located close to a second end of the chamber.
Preferably, the vessel defining the separating chamber is sealingly mounted to the hollow shaft, and an annular space formed between the radially inner part of the chamber and the hollow shaft communicates with the inside of the hollow shaft through at least one aperture through a wall of the hollow shaft so that a pressure at the air outlet of the chamber is maintained lower than the pressure at the inlet and the oil outlet of the chamber.
The air/oil separator according to the present invention advantageously provides an efficient solution with a relatively simple structure for separating oil suspended in air. A major proportion of the oil suspended in the air forms oil droplets in the passages of the packing at a first stage after entering the packing. The major proportion of oil droplets formed in the passages at the first stage will partially block the passages and slow down the continuous movement of a following portion of the mixture in the passages although the oil droplets are eventually ejected away radially by centrifugal forces. The cut-away area formed close to the inlet is used as an early exit from the passages in the packing and a temporary reservoir for the major proportion of the oil droplets so that the oil droplets formed in the passages at the first stage are collected in the cut-away area and quickly ejected from the packing to clear the passages and facilitate a continuous movement of a following portion of the mixture of air and oil.
Other advantages and features of the invention will be better understood with reference to the preferred embodiments described below.